1. Field of the Invention
This invention relates to an altimeter for aircraft, and more particularly, relates to a spread spectrum waveform encoded altimeter apparatus, which has a low probability of being intercepted.
2. Description of the Prior Art
Radio frequency altimeters are known. The most common type of radio frequency altimeter transmits a burst of energy from a transmitting antenna and receives the reflected energy signal back at a receiving antenna. At the time the burst of energy is transmitted a ranging system or counter is started. When the reflected energy signal is received, the ranging system or counter is stopped. The time for making the round trip may be converted into distance or elevation by known techniques.
The same technique has been proposed for use in a radar altimeter; however, such a system suffers from weakness of the reflected signal at higher elevations. Continuous wave (CW) radio frequency altimeters are known, but heretofore have been limited because the transmitted signal is electromagnetically coupled by leakage signals into the receiving antenna and receiving antenna path.
There is need for an accurate continuous wave altimeter. Moreover, there is a need for a radar frequency altimeter, which has a low probability of being detector and/or jammed.